Structure and the composition of species in timberline ecotones of the southern Andes

Abstract

The subalpine southern Andean timberline is characterized by deciduous Nothofagus pumilio forests, which change with increasing altitude to Nothofagus krummholz, built of deciduous N. pumilio and N. antarctica. The current study examines the structure and species composition in timberline ecotones of the southern Andes to assess whether the deciduous timberline in the Andes results as convergent structure of, for example, Betula-timberlines of the Northern Hemisphere. Through a Braun-Blanquet phytosociological approach, we show how the characteristic structure and combination of timberline vegetation in the southern Andes vary in latitudes from 33°S to 55°S and altitudes between 2000 m and 600 m. Timberline composition was distinct at sites and included a variety of assemblages ranging from northern Azaro-Nothofagetalia communities to assemblages of the Adenocaulo-Nothofagetalia and Violo-Nothofagetalia in the South. Krummholz generally occurred in a ten to some hundred meter wide ecotone. Four distinctive N. pumilio growth forms were identified; (1) 'elfin woods' where trees become more and more stunted as they approach the woodland limit readily observed in previous studies; (2) 'cornice-like' growth form characterized by branches close to the ground; (3) restricted to the leeward side, a single-stemmed habit occurred that was characterized by stems aligned downhill before curving up into vertical alignment; and (4) on the wind-exposed side, growth forms were characterized by single-stemmed habit and long branches running uphill. These growth form changes were apparently controlled by changes in abiotic factors and climate rather than genetically determined. In conclusion, the recognition of the deciduous foliage and deformed habit of timberline trees in the southern Andes is explained by extreme climate. Comparison of same latitudes revealed that maximum altitudes of southern Andean timberline were generally higher than those of evergreen Nothofagus limits in New Zealand but lower than timberlines of Holarctic mountain ranges. These results serve to emphasize that understanding structure and physiognomy of the southern Andean timberline and its apparent stability may require attention to ecophysiological adaptations and responses of timberline Nothofagus to high-mountain environment. © 2005 Springer-Verlag Berlin Heidelberg.